The present invention pertains to the field of distillation of fluid mixtures, and in particular to multieffect distillation processes to separate multicomponent mixtures containing three or more components into four product streams each enriched in one of the components.
Multieffect distillation has long been considered as one of the methods to reduce energy consumption in distillation columns. In a multieffect distillation, two heat integrated distillation columns are used. This heat integration is achieved by operating one column at a higher pressure than the other. Feed is fed to one of the distillation columns. The vapor from the top of the high pressure column is condensed by heat exchange with the liquid at the bottom of the low pressure column. Thus, the vapor in the high pressure column provides the boilup duty for the low pressure column.
There are several studies on the use of multieffect distillation for a binary feed distillation, the earliest being that of distilling air to produce nitrogen and oxygen. For this purpose, air is treated as a binary mixture and is fed to a high pressure column. The crude liquid oxygen from the bottom of the high pressure column is fed to an intermediate location of the low pressure column. The bottom of the low pressure column is boiled by condensing the nitrogen vapor from the top of the high pressure column. Both nitrogen and oxygen are produced from the low pressure column.
There are several more multieffect distillation processes for binary distillation. Details of these appear in a paper by Wankat ("Multieffect Distillation Processes", P. C. Wankat, Ind. Eng. Chem. Res., pages 894-905, volume 32, 1993). Tyreus and Luyben studied the multieffect distillation for propylene-propane and methanol-water separations ("Two Towers Cheaper Than One?", B. D. Tyreus and W. L. Luyben, Hydrocarbon Processing, pages 93-96, July 1975). They found that as compared to a conventional single distillation column, the multieffect distillation consisting of two distillation columns required about 46% less steam for propylene-propane separation and about 40% less steam for methanol-water separation.
Recently, multieffect distillation processes for the distillation of ternary feed mixtures have been explored. This is due to an attractive feature of multieffect distillation in reducing the boilup needs from an external heat source.
U.S. Pat. No. 5,245,832 (Roberts) discloses a multieffect distillation process consisting of three distillation columns to separate air into three product streams each enriched in oxygen, argon and nitrogen respectively. The use of three distillation columns and an associated increase in the number of reboilers and condensers make this process unattractive.
Annakou and Mizsey ("Rigorous Comparative Study of Energy-Integrated Distillation Processes", O. Annakou and P. Mizsey, Ind. Eng. Chem. Res., pages 1877-1885, Volume 35, 1996) have proposed four multieffect distillation processes each consisting of two distillation columns to distill a given ternary mixture. These four prior art processes are shown in FIGS. 1 through 4. In these processes, a ternary mixture having components A, B and C (mixture ABC) is separated into three product streams each enriched in one of the components. A is the most volatile component and C is the least volatile component.
In the prior art process of FIG. 1, the ternary feed mixture 100 is fed to an intermediate location of the high pressure column 110. (A location of a distillation column is an "intermediate location" when there is at least one separation stage above and one separation stage below that location. A "separation stage" is a mass transfer contract device between liquid and vapor phases, such as a suitable mass transfer tray or a packed height of a suitable packing.) The vapor stream 184 enriched in component A from the top of the high pressure column is condensed in the reboiler/condenser 116. A portion of the condensed stream 186 provides the reflux for the high pressure column and the other portion provides the A-enriched product stream 190. From the bottom of the high pressure column, a portion of the A-lean binary stream that is enriched in components B and C (stream 140) is recovered as stream 142. The pressure of this stream is reduced across valve 130 and fed to the low pressure column 120. A product stream 198 enriched in component B is produced from the top of the low pressure column, and product stream 194 enriched in the heaviest component C is produced from the bottom of this column. The boilup at the bottom of the low pressure column is provided by vaporizing some bottom liquid in the reboiler/condenser 116. This reboiler/condenser provides thermal linking between the high pressure column and the low pressure column.
The prior art process in FIG. 2 is similar to the one in FIG. 1 in that it also uses a high pressure column 110 and a low pressure column 120 that are thermally linked through a reboiler/condenser 116. However, the feed stream 100 is now fed to an intermediate location of the low pressure column, and the product stream 190 enriched in the most volatile component A is recovered from the top of this column. The A-lean and essentially binary stream 140 containing components B and C is now collected from the bottom of the low pressure column and is increased in pressure across a pump 236 and then fed as stream 144 to the high pressure column 110. The high pressure column produces two product streams (198, 194) each enriched in components B and C respectively.
The prior art process of FIG. 3 is similar to the one in FIG. 1 in that it also uses a high pressure column 110 and a low pressure column 120 that are thermally linked through a reboiler/condenser 116. The feed stream 100 also is fed to an intermediate location of the high pressure column. However, rather than producing an A-enriched product stream from the top of the high pressure column, a product stream 194 enriched in the heaviest component C is produced from the bottom of this column. The vapor stream 350 at the top of the high pressure column is now lean in C but rich in both components A and B. After condensation, a portion of the condensed stream is withdrawn as stream 356, reduced in pressure across a valve 330 and fed to the low pressure column. The product streams (190, 198) enriched in components A and B are produced from the low pressure column.
The prior art process of FIG. 4 also uses a high pressure column 110 and a thermally linked low pressure column 120. The feed mixture is fed to an intermediate location of the low pressure column. The C-enriched product stream 194 is recovered from the bottom of the low pressure column. A mixture lean in C but rich in A and B is recovered as stream 356 from the top of the low pressure column, pumped through pump 436 and fed to the high pressure column. The high pressure column produces the A-enriched and the B-enriched product streams (190, 198).
A common feature of all the prior art processes in FIGS. 1 through 4 is that the ternary feed mixture is fed to one of the two thermally linked columns. In the distillation column, the feed is distilled into a product stream at one end and an essentially binary mixture stream at the other end. The binary mixture is then fed to the other distillation column and a product stream is recovered from each end of this distillation column. In all, three product streams each enriched in one of the components is produced.
It is well known that among the conventional distillation column processes, a fully thermally coupled distillation system requires the lowest heat duty for a ternary distillation ("Minimum Energy Requirements of Thermally Coupled Distillation Systems", Z. Fidkowski and L. Krolikowski, AlChE Journal, pages 643-653, Volume 33, 1987). However, Annakon and Mizsey found that the multieffect distillation column processes of FIGS. 1 through 4 generally required less heat duty than the fully thermally coupled distillation system. The ternary mixtures studied by them are: pentane-hexane-heptane; isopentane-pentane-hexane; and butane-isopentane-pentane. Furthermore, they found that in the cases of relatively pure products, the multieffect distillation process is always the most economical solution.
It is desired to have multieffect distillation processes with even lower heat demand than the prior art multieffect distillation processes for the distillation of a multicomponent feed mixture.
It is further desired to have multieffect distillation processes that are easy to operate while having low heat requirements.
It is still further desired to have multieffect distillation processes using only two distillation columns for the distillation of ternary mixtures.
It also is desired to have multieffect distillation processes which overcome the difficulties and disadvantages of the prior art to provide better and more advantageous results.